1. Field of the Invention
The present invention relates generally to the field of digital video processing and, more particularly, to a system and method for post-processing decompressed motion video sequences where a block-based motion-compensated transform coding technique is used for compression and decompression.
2. Description of the Related Art
Video images can be represented by a digital signal in which a series of bits of information is used to represent each video frame. Where the bandwidth of a particular communication system is limited, such as in the integrated services digital network (ISDN) or the public telephone network, low bit-rate image coding is particularly useful for transmitting visual images and communications. As such, increasing demands have been placed on the use of low bit-rate coding. Low bit-rates between p.times.9.6 kbit/s and p.times.384 kbit/s are most frequently used for low bit-rate transmissions. As demands increase, the picture quality of video images generated through low bit-rate coding becomes critical.
Coded picture quality is generally determined by the type of coding technique used and the targeted bit rate. Coding processes, however, have inherent loss characteristics. Images coding often results in noise or spurious signals upon reconstruction of the image. Noise and spurious signals that occur as a result of such imaging techniques are often referred to as artifacts. Artifacts can often reach a level where they appear in the video image with as much strength as the signals produced by the real objects of the image. Moreover, artifacts often become even more visible for low bit-rate transmissions.
The characteristics of these artifacts depend on the form of coding technique used. Currently, the most well-known and popular low bit-rate coding technique involves block-based motion-compensated transform coding. Such a coding technique is used in many image compression standards, such as the CCITT (Consultative Committee on International Telegraphy and Telephony) Recommendation H.261, Draft Revised Recommendation H.261-Video Codec for Audiovisual Services at p.times.64 kbit/s, Study Group XV-Report R95 (May 1992), herein incorporated by reference, and the more recent proposals arising out of the Telecommunication Standardization Sector Study Group 15, Working Party 15/1 Expert's Group on Very Low Bitrate Videophone (LBC-93), herein incorporated by reference.
This CCITT standard, although an image compression standard often used for multimedia applications, produces highly noticeable artifacts upon reconstruction of an image at low bit-rate. Those artifacts are often referred to as "blocking effects", "quantization noise" and the "mosquito phenomenon". Future standards using block-based motion-compensated transform coding will likely also produce such artifacts.
"Blocking effects" are spatial domain distortions which appear as discontinuities on the edges of an image and which yield average values of luminance and chrominance across a block boundary, a so-called spatial artifact. These distortions are caused by using different coding parameters for adjacent blocks. For blocks containing edges, edges can be discontinuous on block boundaries since each block is coded independently of its neighbors. Similarly, for monotone blocks where the intensity in the original image changes gradually, the intensity in the coded blocks can change abruptly from one block to another, due to the different quantization parameters used for each block.
"Quantization noise" is a distortion caused by quantization processes. When a quantization step size is small, the distortion caused by the quantization is called granular noise, a noise with a high spatial frequency that is evenly distributed over an entire block. This noise is independent of the signal. When a quantization step size is large, the quantization noise is signal-dependent since the signal can be all mapped to zero. Where large quantization forces the high frequency components to zero, the artifacts can be described as "noise contour". Various coefficients used in Discrete Cosine Transform (DCT) techniques may cause this distortion to spread over the entire block when the edge block is transformed into the DCT domain, particularly in view of the fact that DCT techniques are inefficient in representing edges compared with flat areas. Significant losses in edge information and the creation of speckle-like distortions near the edges of a particular block may occur as a direct result of coarse quantization of the DCT coefficients.
The "mosquito phenomenon" is a high frequency granular type of noise, which appears similar to a moving cloud of insects, that is caused by motion compensation and quantization processes. This phenomenon is a distortion which appears in the temporal domain, a so-called temporal artifact. These artifacts further degrade reconstructed picture quality.
Therefore, to improve the reconstructed picture quality, postprocessing to reduce the aforementioned artifacts is frequently necessary.
There have been many nonlinear postprocessing techniques proposed for reducing coding artifacts and for improving reconstructed picture quality. For example, in an article published by B. Ramamurthi and A. Gersho, "Nonlinear Space-Variant Postprocessing of Block Coded Images", IEEE Trans. on Acoustics, Speech, and Signal Processing, VOL. ASSP-34, No. 5, October 1986, the authors proposed a space-variant nonlinear postprocessing technique.
Similarly, in articles published by R. L. Stevenson, "Reduction of Coding Artifacts in Transform Image Coding", Proceedings of the International Conference on Acoustics, Speech, and Signal Processing, 1993; and A. Zakhor, "Iterative Procedures for Reduction of Blocking Effects in Transform Image Coding", IEEE Transactions on Circuits and Systems for Video Technology, Vol. 2, No. 1, March 1992, pp. 91-95, Lee et al., "Postprocessing of Video Sequence Using Motion Dependent Median Filters," SPIE Vol. 1606 Visual Communications and Image Processing '91, Image Processing, July 1991, among others, the authors proposed various techniques for improving picture quality.
However, the aforementioned proposed techniques were designed to reduce artifacts produced by either still image coding techniques, where no motion compensation is employed, or other coding techniques which do not utilize block-based motion-compensated transform coding.
In addition, in articles published by A. Nieminen, P. Heinonen, and Y. Neuvo, "A New Class of Detail-Preserving Filters for Image Processing", IEEE Trans. on Pattern Analysis and Machine Intelligence, VOL. PAMI-9, No. 1, January 1987, and P. Heinonen, "FIR-Median Hybrid Filters", IEEE Trans. on Acoustics, Speech and Signal Processing, VOL. ASSP-35, No. 6, June 1987, various filtering techniques have also been proposed.
Different coding techniques produce different artifacts. As such, motion-compensated coding techniques often introduce different spatial and temporal artifacts on the reconstructed picture. Techniques proposed by these various authors are unnecessarily complex and often fail to account for the different spatial and temporal artifacts produced by block-based motion-compensated transform coding. As such, the various techniques proposed to date are not particularly efficient for eliminating spatial and temporal artifacts from decompressed signals generated by block-based motion-compensated transform coding.